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The presence of THC in blood is poorly correlated with behavioral impairment, according to data published in the Forensic Science International.
Norwegian researchers assessed the relationship between drug concentrations and impaired psychomotor performance in a cohort of over 15,000 individuals suspected of drugged driving and 3,684 drug-free controls. Driving performance was assessed by participants’ performance on a clinical test of impairment (CTI).
Consistent with prior literature, “The correlation between drug concentration was high for ethanol, … but low for THC.”
Specifically, authors determined, “For THC, the median drug concentrations changed little between drivers assessed as not impaired and impaired.”
They concluded, “The lack of a close relationship between drug concentration of THC and degree of impairment at the individual level is in accordance with several observations from experimental studies [where participants engaged in the] controlled intake of cannabis.”
The authors’ findings are consistent with those of numerous studies reporting that neither the detection of THC nor its metabolites in blood or other bodily fluids is predictive of impaired driving performance. As a result, NORML has long opposed the imposition of per se THC limits for motorists and has alternatively called for the expanded use of mobile performance technology like DRUID.
Full text of the study, “The relationship between clinical impairment and blood drug concentrations: Comparison between the most prevalent traffic relevant drug groups,” appears in Forensic Science International. Additional information is available from the NORML Fact Sheet, ‘Marijuana and Psychomotor Performance.’
